What upgrades do I have to buy to enable the plugins?

It's written that "Most WordPress.com users won’t find a Plugins tab in your dashboard. We include and configure the plugins for you, so you don’t need to worry about them."
We need plugins like SEO (http://wordpress.org/extend/plugins/all-in-one-seo-pack/).
I'm a free user so I don't know how to enable plugins. Please contact me as soon as possible, thanks!

There is no FTP access to free hosted WordPress.com blogs and we bloggers cannot install any plugins. To have plugins you need to hire a web host and set up your own self hosted WordPress.org install. http://support.wordpress.com/com-vs-org/
There is no upgrade you can purchase that changes this.

Here's your answer: you can't use plugins. Period. You might be able to if you were in the $2600 per month VIP program, but otherwise no. Plugins are not for WordPress.COM blogs; they are for WordPress.ORG blogs.

By the way, I have some of each and the built in SEO advantages of WordPress.com vastly outweigh any SEO plugins. That particular one is just awful. Perhaps it was good once, but the day I disabled it, my hits tripled.